1. Field of the Invention
The present invention relates to a shading correcting device that performs shading correction on an image signal and an imaging apparatus.
2. Description of the Related Art
In recent years, there is an increasing demand for a compact and low height image sensor as a solid-state image sensor, such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS), mounted on a digital still camera, a mobile phone, or the like, along with the miniaturization of electronic devices. However, because a distance between an optical system and an image sensor becomes narrow due to the low height, an incident angle of light entering the image sensor becomes small in a peripheral part of the image sensor compared with the center of the image sensor. Light has a property that a refractive index when passing a lens, an infrared light cut filter, or the like increases in proportion to the wavelength of the light. Therefore, if the incident angle of light entering the image sensor is small, the position at which the light is focused changes depending on the wavelength of the light. Consequently, color shading occurs on an output image from the image sensor, so that coloring is recognized on the peripheral part of the image sensor.
Japanese Patent Application Laid-open No. H08-79773 discloses a shading correcting device, in which color shading is corrected by approximating a correction coefficient corresponding to coordinates (x,y) of an image sensor by the following N-dimensional surface function (N is an integer equal to or larger than two) for each color:N-dimensional surface function: f(x, y)=a01×y+a02×y2+a10×x+a11×y+a12×xy2+a20×x2+a21×x2y+a22×x2y2+ . . . +aN−1N−1×xN−1yN−1 
Japanese Patent Application Laid-open No. 2006-246196 discloses a shading correcting device, in which block correction values of part of blocks out of block correction values for correcting an image in correcting process unit (block) is stored and block correction values of other blocks are calculated based on the stored block correction values. Then, luminance values of pixels composing an image are corrected based on the stored block correction values and the calculated block correction values.
However, because of downsizing of a pixel size, there appears a problem that color shading occurs due to a layout of pixels. The technology disclosed in Japanese Patent Application Laid-open No. H08-79773 can only perform the correction expressed by the N-dimensional surface function, so that coloring shading due to the layout of pixels cannot be corrected accurately.
The technology disclosed in Japanese Patent Application Laid-open No. 2006-246196 only corrects the luminance values of pixels, so that color shading cannot be corrected efficiently. Furthermore, if the color shading correction is performed, correction values of four colors of, for example, red, green, blue, and green need to be stored, which leads to increase in size of a circuit configuration.